The nasal mucosa is the initial site for detoxifying airborne pathogens, irritants, allergens, and toxins. Analysis of nasal secretions has provided important insights into the mechanisms by which mucous membranes respond to these encounters. In secretions, there are a variety of proteins playing roles in protecting the underlying epithelium. For instance, neutral endopeptidase (NEP) is secreted in order to metabolize neuropeptides which might otherwise stimulate the epithelium. Secretory leucoprotease inhibitor (SLPI) is a major secretory protein which acts to prevent neutrophil elastase injury to the epithelium. Provocation of allergic individuals with allergen led to the secretion of three neuropeptides (SP, CGRP, and VIP), despite the presence of NEP. Aspirin challenge of asthmatics led to the secretion of glandular products in response to dramatically increased production of leukotrienes.